Render subsection of frame
Blender will only render the scene as it exists in one specific frame, meaning you can't change frames half way through a render. While you can use the compositor to crop half of the image out, the entire image will be rendered before the crop happens so no time will be saved.

The only way I can think to reduce the render time is to cut the horizontal resolution in half. That would also mean moving the camera to compensate for the altered resolution, two cameras could be parented to stay in the same relative position so that both renders match up. Camera one can then be used to render frames 1-50 and camera two for 51-100. The two "half" images can then be composited together to get the final result.